Electrical apparatus with thermoelectric gas drying



Feb. 6, 1968 B FRANKUN 3,367,120

ELECTRICAL APPARATUS WITH THERMOELECTRIC GAS DRYING Filed Nov; 29, 1965 4 SheetsSheet 1 PRESSURE SENSING DEVICE CHANGE- OVER" sw.

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D.c. SUPPLY OPERATING MEANS INVENToR Edwina Bea umohT Frat; K

ATToR Feb. 6, 1968 E. B. FRANKLIN 3,367,120

ELECTRICAL APPARATUS WITH THERMOELECTRIC GAS DRYING Filed Nov. 29, 1965 4 Sheets-Sheet 3 B L; is

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D.C.SUPPLY CHANCE- OVER SWITCH TIME 53 SWIITCH SHORT- clRculTlNcs SWITCH FIG.3

INVENToR Edwar 'BeaumohT fian [fl/ 0 ATToRN Ys' Feb. 6, 1968 Filed Nov. 29, 1965 E. B. FRANKLIN ELECTRICAL APPARATUS WITH THERMOELECTRIC GAS DRYING 4 Sheets-Sheet 4 INVENTOR EA wave Beau man T FrahKh'h A'ITo United States Patent 3,367,120 ELECTRICAL APPARATUS WITH THERMO- ELECTRIC GAS DRYING Edward Beaumont Franklin, Stafford, England, assignor to The English Electric Company Limited, London, England, a British company Filed Nov. 29, 1965, Ser. No. 510,236 Claims priority, application Great Britain, Dec. 11, 1964, 50,494/64 7 Claims. (til. 62-3) ABSTRACT OF THE DISCLOSURE An electrical apparatus such as a transformer including a gas space of which the moisture content is required to be maintained at a low level, the apparatus including a surface with which the gas in the gas space comes into contact and which is cooled by thermoelectric elements to below the dew-point of the gas so that the gas is dried by the deposition of moisture. Advantageously, the surface is provided by a segmented cooling duct which forms part of a closed circuit gas path around which gas in the gas space is convectively circulated so that a progressive drying effect is achieved. A gas passage connected to the closed circuit gas path allows the apparatus to breathe by connecting the gas space intermittently to atmosphere.

This invention relates to electrical apparatus particularly transformers, including an enclosure defining a gas space from which it is required to remove moisture.

According to one aspect of the invention electrical apparatus is provided with gas drying means which includes thermoelectric elements for drying gas within the apparatus by cooling the gas to below its dewpoint.

According to a second aspect of the invention moisture condensed out of the gas is cooled to below its freezing point by the gas drying means thereby to hold the moisture captive in the form of ice.

In order that the invention may be more clearly understood four forms of gas drying means according to the invention and incorporated in an oil filled electrical transformer having an oil conservator tank will now be described, by way of example, with reference to the accompanying diagrammatic drawings which show the gas drying means as comprising a drying device and associated control equipment. In the drawings:

FIG. 1 shows the first form of gas drying means in section, with its associated control equipment and the sectioned oil conservator tank of the transformer,

FIG. 2 is a section through the gas drying means along the line A-A of FIG. 1,

FIG. 3 shows the second form of gas drying means and associated equipment,

FIG. 4 shows the third form of gas drying means and associated equipment,

FIG. 5 shows in section the fourth form of gas drying means, and,

FIG. 6 is a section through the gas drying means along the line BB of FIG. 5,

Referring now to FIGS. 1 and 2, the gas drying device shown generally at 10 communicates with the gas space above the oil surface of the transformer conservator tank 11 by a pipe 12 a part 13 of which is immersed in the oil and which also serves to support the drier 10.

The oil in the conservator tank communicates with oil in the transformer tank (not shown) through the pipe 32.

The device 10 has a metal casing 14 comprising a top plate 15 which is connected to the pipe 12, a bottom plate 16, and two pairs of opposing side plates 18 arranged in a square as shown in FIG. 2. The top and hot- 3,367,1Zll

Patented Feb. 6, 1968 tom plates are of double walled construction having lagging between the walls, but the two side plates 18 are metal plates having fins 19 welded to their outsides.

Attached to the inside of each side plate 18, are a number of thermoelectric elements as shown at 20.

The thermoelectric elements 20 are spaced apart circumferentially and longitudinally of the drying device and have attached to their inner faces two fin members 21, these latter having cooling fins 22 which extend inwardly into the drying device.

The elements 20 are electrically interconnected by connections (not shown) and are connected to insulatingly mounted terminals 23 and 24 mounted on the casing of the drying device.

The terminals 23 and 24 are connected to a source of direct current voltage 25 through a changeover switch 26 so that the polarity of the supply to the elements 20 may be reversed.

Control of the changeover switch 26 is in response to signals from a pressure sensing device 27 mounted at the top of the conservator tank 11, and switching is effected by an associated control means 27A.

Also operable in response to signals from the pressure sensing device 27, through the control means 27A, are valve operating means 28 and 29 associated with gas inlet and outlet valves 30 and 31 respectively.

In operation of the transformer air will be drawn into the transformer conservator during load reduction of the transformer and also gas will be evolved in the transformer and this gas will pass to the top of the conservator tank via the pipe 32. The gas drying means operates, as

is described below, to dry the evolved gas, the air drawn into the conservator tank, and also any air already in the conservator. By this operation the drying means maintains the moisture content of the winding insulation of the transformer at an acceptably low level.

If, after a period of transformer operation, the electrical load on the transformer is reduced, then a negative pressure difference is set up between the inside of the conservator and atmosphere by the consequent fall of the oil level in the conservator. When this pressure difference reaches a certain value the pressure sensing device provides a signal to open the inlet valve 30 thereby allowing air to enter the conservator via the drying device and the pipe 12, the outlet valve 31 being meanwhile maintained closed.

The direct current supply to the thermoelectric elements 20 however is of such polarity that the elements transfer heat from the cooling fin members 21 to the side plates 18 where it is dissipated to atmosphere by the fins 19. The correspondingly depressed temperature of the cooling fin members 21 causes air and gas in the conservator to be cooled to below its dewpoint. Water vapour carried by the gases is therefore deposited as water on the surfaces of the members 21 and this water is then further cooled until it forms ice which is thus held captive.

As thermal equilibrium of the transformer is approached the pressure difference between the inside of the conservator and atmosphere becomes smaller until, at a predetermined value, the pressure sensing device 27 effects closure of the air inlet valve 30 thereby preventing air from entering the conservator during periods of constant load on the transformer.

Whilst such conditions exist and the valves 30 and 31 are both closed, the gas drying device effects the progressive drying of the gases in the conservator by setting up a water vapour pressure gradient down the pipe 12 thus maintaining the water vapour pressure in the conservator at a low level.

Defrosting of the gas drying device occurs when the transformer is subjected to an increased electrical load when the positive pressure set up in the conservator reaches a sufliciently large value to cause the pressure sensing device 27 to open the outlet valve 31. Gases from the conservator then flow to atmosphere through the valve 31 passing the ice-carrying cooling fin members 21 and the heat which is picked up during passage through the immersed part 13 of the pipe 12 causes the ice on the members to melt. The water thus formed drops off to leave the drier through the valve port of the outlet valve 31.

In order that refrosting may be effected as quickly as possible, further heat is provided to the fin members 21 by arranging that, in addition to opening the outlet valve 31, the pressure sensing device 27 also effects reversal of the DC. supply to the elements 20 by operating the changeover switch 26, thereby reversing the direction of heat flow through the elements 20 and so providing heat to the fin members 21.

Defrosting of the gas drying device is continued until the transformer again approaches thermal equilibrium and the pressure difference has reacheda certain low value when the pressure sensing device provides a signal to close the outlet valve 31 and to return the changeover switch 26 to its original position thereby again causing cooling of thecooling fin members 21.

The inlet and outlet valves 30 and 31, in addition to sealing off the transformer during conditions of approximate thermal equilibrium, also serve to provide an appreciable resistance to gas fiow past them, and by so doing they reduce the sensitivity required of the pressure sensing device 27.

FIG. 3 shows the second form of gas drying means which differs from the one described with reference to FIGS. 1 and 2 in that no drying is provided for air as it enters the conservator.

Referring to FIG. 3 the gas drying device is shown generally at 40 and is mounted inside the transformer conservator tank 41. Shown at 42 is one of a number of thermoelectric elements which are mounted on an angle piece 43 itself supported from the wall of the conservator.

Attached to the bottom of the thermoelectric elements 42 is a copper plate 44 to the underside of which is brazed an inverted copper cone 45 below which a funnel 46 is supported by a bracket 47.

Connected across the thermoelectric elements 42 by means of two terminals 48 mounted in the wall of the conservator is a directcurrent supply source 49 which is reversible by a changeover switch 51 which is operated in response to signals from a time switch 50.

The funnel 46 has an outlet orifice 52 associated with which is a valve 53 having a cone-shaped head 54 for allowing the orifice to be sealed. The head 54 is carried on a shaft 55 which has also attached thereto two spaced collars 56.

A bimetallic strip 57 having a hole through which the shaft 55 passes is sandwiched between the two collars 56 and is loosely held in position by the engagement of each of its ends in a groove in oneof two insulating electrically retaining blocks 58 attached to the inside of the funnel.

At the bottom of the funnel 46 is attached a tube 60 which passes out of the conservator through a side wall as shown. Attached to the bimetallic strip 57 near its two ends are terminals 61 by which the strip is connected (by connections not shown in series with the thermoelectric elements, the strip and elements being connected in series across the voltage source 49 through the. terminals 48 and62. Connected across the terminals 62 on their outside is a short circuiting switch 63 which is operable by signals from the time switch 50.

Oil in the conservator tank communicates with oil in the transformer tank (not shown) through a pipe 59. i

In operation the gas drying means operates, as has previously been described, to dry the gases in the transformer conservator, except that operation is in response to the time switch 50 and occurs independently of the load on the transformer. The time switch 50 is arranged normally to effect cooling of the cone 45 by the elements 42, the heat removed from the cone being dissipated to atmosphere by conduction via the angle piece 43 to the wall of the conservator tank, By thus cooling the cone, gas in its vicinity is cooled and is therefore dried by the depositing of moisture on the cone. The moisture deposited on the cone is frozen and thus held captive.

Defrosting is also controlled by the time switch 50 which operates the changeover switch 51 to reverse the polarity of the supply 49 to the thermoelectric elements 42. At the same time time switch 50 causes opening of the short circuiting switch 63 thereby opening the water outlet valve 53 by allowing the load current of the elements 42 to pass through the bimetallic strip 57, the heat thus generated in the strip causes it to arch upwardly at its middle to lift the shaft 55 by the top collar 56 so as to open the outlet valve 53.

Water liberated from the cone thus drips into the funnel 46 and then leaves the conservator through the tube 60 via the orifice 52 of the open valve 53.

After a time sufficient for defrosting to have taken place the time switch again operates to initiate the gas drying sequence by closing the short circuiting switch 63 and by again reversing the polarity of the DC. supply to the elements 42.

It should be noted that compared with the duration of time during which gas drying takes place, the time switch 50 is set so that defrosting is only of short duration, and any increase in moisture content of the insulation of the transformer windings due to undried gases in the conservator is therefore negligible.

The third form of gas drying means as shown in FIG. 4 operates and is controlled electrically in the same manner as the gas drying means of FIG. 3 and in FIG. 4 like reference numerals indicate like parts of FIG. 3. This third form of the invention has a closed circuit gas drying device which effects progressive drying. The gas drying device is shown generally at and is supported from the conservator 81 of the transformer (shown in part at 82) by two pipes 83 and 84 which connect the drying device with the gas space in the conservator the oil in which communicates with. the transformer oil through a pipe 107.

The casing 85 of the gas drying device has the pipe 83 connected at its top 86 and the pipe 84 connected at one of its sides 87.

Disposed. circumferentially and longitudinally of the gas drying device along part of its length are thermoelectric elements as indicated at 89 and these elements are sandwiched between the inside surface of the casing 85 and the outside of an open ended structure 90 formed of four plates arranged in a square.

Disposed around the gas drying device on the outside and corresponding in extent to the elements 89 are cooling fins 91 secured to the casing 85.

Inside the structure 90 is a copper tube 92 arranged in a helix which is attached to the structure 90 at its points of contact therewith. The tube 92 communicates through a thermally insulating pipe 93, with a valve chamber 94 in which are formed the valve ports associated with inlet and outlet gas valves 95 and 96.

The valves 95 and 96 are provided with valve operating means shown diagrammatically at 97 and 98 respectively and the operating means are controlled by a control means 99A in response to signals from a pressure sensing device 99 mounted on the wall of the conservator.

To the inside of the helix is attached an open-ended copper cylinder 100 having a tapered end 101, and the cylinder 100, the helix, the structure 90, and the inside faces of the elements 89 are arranged to be in close thermal contact with one another.

The free end of the tube 92 is bent inwardly of the drier through a hole formed in the wall of the cylinder 100 as shown at 108.

Below the tapered end 101 of the cylinder is mounted a funnel 46 having a water outlet valve 53 and a bimetallic strip 57 as previously described with reference to FIG. 3. Under the orifice 102 of the funnel is a water container 103 supported on an externally threaded flange 104 Welded to the drier casing.

In operation the alternate drying and defrosting cycles are initiated by the time switch exactly as described with reference to FIG. 3.

Independently of the electrical operation governed by the time switch 50, a pressure sensing device 99 effects opening and closing of the inlet and outlet valves 94 and 95 respectively exactly as described with reference to FIGS. 1 and 2 and by so doing allows the conservator to breathe.

During a drying cycle, air enters the conservator and moisture from the indrawn air is deposited as ice on the inside surface of the cooled tube 92, and the air is thereby dried.

When, however, steady state conditions of the transformer apply and the inlet and outlet valves 95 and 96 are both closed, then progressive drying of all gases in the conservator occurs as the gases flow in the closed circuit comprising the conservator 81, the pipe 83, the gas drying device 80, and the pipe 84 back to the conservator, the direction of flow being as shown by the arrows. Moisture is thus progressively removed from the gases and held captive as ice on the inside surface of the cylinder 100.

A certain amount of moisture is deposited on the inside surface of the tube 92 by gas exhaled from the conservator. However, the amount of moisture involved is small and has little effect on efficiency.

Water liberated from the tube 92 and the cylinder 100 during defrosting runs down through the orifice 102 into the water container 103.

The fourth form of gas drying means as shown in FIGS. 5 and 6 resembles the form of gas drying means described with respect to FIG. 4 in that it has a closed circuit gas drying path whereby it effects progressive gas drying during substantially constant load conditions of operation of the associated transformer.

In FIG. 5 the gas drying means is shown as including a drying device which is shown in central vertical section generally at and supported from the conservator (not shown) of the associated transformer by two pipes 112 and 113 which connect the gas drying means with the gas space in the conservator in an arrangement substantially similiar to that described with reference to FIG. 4.

The gas drying device 110 has a casing 116 which is generally of hollow square section and has longitudinal cooling fins such as are shown at 117 disposed around its periphery. On one of its sides the casing wall is considerably thickened to form a rounded part 118. To permit easy inspection and maintenance of the gas drying means the casing 116 is divided into two parts with a gasket 119 therebetween. The two parts are denoted by the reference numerals 120 and 121 and are secured together by two bolts 122 which extend the length of the casing at either side thereof. At their ends the bolts 122 co-operate with nuts of which one is shown at 123.

Connected at the top of the casing 116 are the two pipes 112 and 113, and the pipe 113 communicates with a passage 124 formed longitudinally in the casing in the rounded part 118. At the bottom of the gas drying device the passage 124 turns inwardly and communicates with a thermally insulating pipe 125 secured to the inner surface of the casing wall.

Extending longitudinally down the drying device and connecting the pipe 112 with the thermally insulating pipe 125 is an open ended cooling duct 126 which is made up of a number of duct pieces 127 of hollow section arranged end to end but spaced apart as is explained later. Each duct piece 127 has formed therein a number of parallel and longitudinal webs one of which is indicated at 128 in FIG. 6.

Spaced longitudinally of the casing and disposed between the cooling duct 126 and the inner surface of the casing at the rounded part 118 are thermoelectric elements 130 each of which is individually clamped to the casing by a thermally insulated clamp 131 which passes through the associated element 130. In addition, each thermoelectric element is individually secured, preferably by brazing, to one of the duct pieces 127, and therefore serves to support the associated duct piece in position spaced from each adjacent duct piece.

The elements 130 are electrically interconnected by means (not shown) and are connected to insulatively mounted terminals 132 mounted in a terminal box 133 formed on the casing of the gas drying device as shown in FIG. 5.

Communicating with the central duct piece 127 of the duct 126 is a breather pipe 134 which extends to the bottom, as shown in FIG. 5, of the gas drying device, and which communicates with atmosphere via non-return inlet and outlet gas valves formed in a chamber 135. Of the two gas valves only the outlet gas valve is shown, being denoted generally by the reference numeral 136.

Supported below the bottom open end of the cooling duct 126 is a copper funnel 137 which is connected via an S shaped pipe 138 to one end of a water outlet pipe 139.

At its other end the water outlet pipe 139 is disposed in a recess 141 formed on the under side of the gas drying device and communicates with atmosphere through a protective gauze screen 142 covering the recess.

A second protective gauze screen 143 similarly covers another recess 144 in which the inlet and outlet gas valves are disposed.

The interior of the casing 116 is filled with moisture non-absorbent insulation which provides thermal insulation between the various parts of the gas drying device and also serves to cushion the parts against vibration and shock transmitted from the associated transformer.

In operation the gas drying device 110 operates in a similar way to that of FIG. 4 progressively to dry the gases in the conservator during steady state conditions of the associated transformer and also to dry air entering the transformer on load reduction thereof.

A time switch, together with associated changeover switch and a direct current voltage supply (none of which are shown) are arranged as is shown in FIGS. 3 and 4 and are connected to the terminals 132 to effect alternate drying and defrosting cycles exactly as is described with reference to those figures.

The non-return inlet and outlet gas valves are biased slightly to their closed positions and operate directly in response to the pressure differential produced across them during transformer load increase and transformer load reduction. They therefore allow the transformer to breathe independently of the time switch.

During a drying cycle transformer load reduction causes air to be drawn into the conservator through a branched gas path having two branches each of which includes one half of the cooling duct 126 and the pipe 112 or 113. Air drawn into the conservator through this gas path deposits moisture as ice on the inside surface of the cool ing duct and is thereby dried.

When the transformer is operating substantially on constant load during a drying cycle the inlet and outlet gas valves are both closed and gas is convectively circulated in the closed gas path comprising the pipe 112, the gas space in the conservator, the pipe 113, the channel 124, the pipe 125, and the segmented cooling duct 126. Gas flowing in this path passes through the gas drying device as is indicated by the arrows in FIG. 5 to be freeze dried in the cooling duct 126. Progressive drying of the gas is thereby effected.

In this embodiment of the invention heat for providing the convective gas flow is derived from the hot faces of the thermoelectric elements 130 by thermal transmission to the channel 124.

During a defrosting cycle the direction of heat flow through the thermoelectric elements is reversed and .ice in the cooling duct melts and captive moisture leaves the drying means by the water trap provided by the S shaped pipe 138.

In this respect it will be noted that no water outlet valve is provided for the gas drying means of FIGS. and 6. Air is prevented from entering the conservator via the Water outlet tube 139 by moisture trapped in the U shaped part of the pipe 138.

Preferably the pipe 138 is cooled to below the freezing point of water during a cooling cycle so that ice so formed provides the necessary gas seal during such a cycle.

The segmented arrangement of the cooling duct 126 allows the duct material to expand and contact due to temperature variation thereof and also prevents shear stresses from being imposed on the thermoelectric elements.

By providing a transformer conservator with any of the various drying means described, water vapour pressure of the space above the'oil in the conservator is maintained at a low average level and moisture produced by the insulation of the transformer, and that previously absorbed by the insulation is therefore continuously removed by transference through the oil to the gas space in the conservator. In this way the moisture content of the winding insulation is maintained at an acceptably low level, thereby ensuring adequate insulation resistance levels in the transformer and improving the aging properties of the insulation.

Various alternative arrangements of the present inven provided for reversing the polarity of the supply to the thermoelectric elements in any of the embodiments described. Defrosting may be effected by interrupting the supply and allowing the cooling surfaces of the drier to heat up naturally.

Also, as might be possible for the embodiment shown in FIG. 1, defrosting of the gas drying means may in some arrangements be effected by the heat carried by air expelled from the conservator, the thermoelectric elements being connected directly to the direct current supply source.

The gas drying means can be controlled Wholly by a device such as the pressure sensing device shown in FIGS. 1 and 4 which operates in response to changes in oil or transformer temperature, oil density, oil surface level or any suitable parameter of the transformer.

Alternatively, or additionally the gas drying means may be controlled in response to parameters not directly related to the transformer load cycle;-amongst these are included time elapsed (as for the gas drying means shown in FIGS. 3 to 6) and ambient temperature.

If desired control may be effected manually after visual inspection.

Although the gas drying means described with reference to the drawings are all arranged to hold moisture captive as ice, if desired they could be arranged to operate at a temperature at which freezing did not occur and the Water thus captured could, for instance, be allowed to leave the drying means through a water trap such as is shown in FIG. 5.

With such an arrangement it would not be essential to provide a period during which the normally cooled parts of the drying means were allowed, or caused, to heat up, and no switching means in the electrical supply to the thermoelectric elements need then be provided.

Other kinds of transformer to which the thermoelectric gas drying means may be applied include the kind having no conservator tank but having a gas cushion above the oil surface in the main transformer tank, or the kind in which the windings are not oil-immersed but are cooled by a gas passing around the windings thereof, the latter being commonly called dry-type transformers.

With thesetypes of transformer the gas drying means according to the invention is fitted to the transformer tank to dry any gas therein and air which is allowed to enter the tank.

Other electrical apparatus to which the thermoelectric gas drying means may be applied include, for example, an electrical reactor, or a switchgear cubicle, in which condensation on the walls, and particularly the roof, can often cause trouble.

What I claim as my invention and desire to secure by Letters Patent is:

1. An electrical apparatus comprising an enclosure which defines a gas space,

a gas drier for progressively drying gas within said gas space,

a cooling duct included in said gas drier,

first conduit means connected between said gas space and one end of said cooling duct,

second conduit means connected between said gas space and the other end of said cooling duct,

a gas drier including a plurality of thermoelectric elements having a cold portion and a hot portion when energized with a direct voltage and being disposed with their cold portions in heat conductive relation with said cooling duct and with their hot portions in heat conductive relationship with atmosphere, and the gas space, the first conduit means, the cooling duct and the second conduit means in combination providing a closed circuit gas path around which gas within the gas space is convectively caused to flow whereby the gas drier is effective progressively to dry the gas in the gas space,

a gas passage connected intermediate the said ends of the cooling duct for connecting the closed circuit gas path intermittently to atmosphere, the cooling duct thereby acting both to dry gas within the gas space and also to dry gas entering the gas space from atmosphere.

2. An electrical apparatus according to claim 1, wherein the cooling duct comprises a plurality of tubular members coaxially arranged and spaced longitudinally apart from one another and each having one said thermoelectric element in heat conductive relation therewith.

3. An electrical apparatus according to claim 2, where in the tubular members of the cooling duct are supported, at least in part, by their respective thermoelectric elements.

4. An electrical apparatus according to claim 1, wherein the cooling duct is vertically disposed and the gas.

drier includes, for forming a part of the closed circuit gas path in series with the cooling duct, a further duct which is thermally insulated from the cooling duct and.

is vertically disposed in heat conductive relation with the hot portions of the thermoelectric elements, the cooling duct and the further duct being therefore respectively effective to cool and heat gas within the gas drier and, in combination, to cause gas to flow convectively around the closed circuit gas path.

5. An electrical apparatus according to claim 1, which comprises a housing within which the cooling duct and the thermoelectric elements are supported, fins formed on the outside of the housing for dissipation to atmosphere of heat removed from the cooling duct and material within the housing for providing thermal insulation and for preventing damage by mechanical vibration.

6. An electrical apparatus comprising an enclosure which defines a gas space,

a gas drier for progressively drying gas within said gas space,

a cooling duct included in said gas drier,

first conduit means connected between said gas space and one end of said cooling duct,

second conduit means connected between said gas space and the other end of said cooling duct,

the gas drier including a plurality of thermoelectric elements having a cold portion and a hot portion when energized with a direct voltage and being disposed with their cold portions in heat conductive relation with said cooling duct and with their hot portions in heat conductive relationship with atmosphere, and the gas space, the first conduit means, the cooling duct and the second conduit means in combination providing a closed circuit gas path around which gas within the gas space is convectively caused to flow whereby the gas drier is effective progressively to dry the gas in the gas space, and a gas passage included within the gas drier and connected to the closed circuit gas path for connecting the path intermittently to atmosphere, the gas passage being disposed in heat conductive relation with the cold portions of the thermoelectric elements for drying gas entering the gas space from atmosphere. 7. An electrical apparatus according to claim 6, wherein the gas passage comprises a coiled tube which surrounds, and is in heat conductive relation with, the cooling duct, the thermoelectric elements being disposed at the outside of the coiled tube with their cold portions in heat conductive relation therewith.

References Cited UNITED STATES PATENTS Fredericks 62-408 Williams 62407 Shipley 6290 Rabe 62408 De Chamerlat 62285 Clancy 6289 Becket 623 Belton 623 Gilman 623 Schmerzler 623 Boehrner 623 Roane 623 Begley 623 Newton 623 Neild 623 Mole 623 Newton 623 Newton 623 WILLIAM J. WYE, Primary Examiner. 

